In the field of receiver apparatuses for communication appliances, for example, it is known practice to use limiting amplifiers. A limiting amplifier is typically implemented by means of a plurality of cascaded amplifier stages. In this way, it is possible to achieve a limiting amplifier characteristic in which smaller input signals are subject to higher gain than larger input signals. Typically, the amplifier characteristic of a limiting amplifier is essentially logarithmic. An example of a limiting amplifier with cascaded amplifier stages is described in “A 2-V 10.7-MHz CMOS Limiting Amplifier/RSSI”, Po-Chiun Huang et al., IEEE Journal of Solid-State Circuits, vol. 35, No. 10, October 2000.
Realization of the limiting amplifier characteristic requires the various amplifier stages each to provide a well-defined gain. However, this is made more difficult by the fact that the amplifier circuits used typically react sensitively to variations in process conditions, supply voltage and temperature conditions and the gain actually provided by an amplifier stage can therefore differ from a desired setpoint value.
In order to provide a particular gain for an amplifier stage, there are various possibilities that exist in principle. One is to implement various ratios for transconductances by CMOS transistors (CMOS: “Complementary Metal Oxide Semiconductor”). A further possibility is to use a transconductance amplifier in connection with a particular load resistor.
In the case of the first option cited, the gain is determined by ratios of transconductances in the transistors used in the amplifier circuit. These ratios can be realized by using transistors having different channel widths. A disadvantage of this option is that the transistors need to be operated in the range of strong inversion, since otherwise their transconductance does not scale with the channel width, as required. In addition, a minimal current is required in order to operate a transistor in the range of strong inversion, which means that achieving low power draw is made more difficult.
In the case of the second option cited, the gain is defined by the product of the transconductance provided by the transconductance amplifier and the resistance value of the load resistor. It is therefore possible to stipulate the gain by means of an appropriate selection of the resistance value of the load resistor. Since operation in the range of strong inversion is not required in this case, achieving low power draw is simplified. However, since both the transconductance and the resistance value of the load resistor are typically dependent on temperature, supply voltage and process conditions, the gain is also subject to corresponding variations, which means that observing a well-defined gain value is made more difficult.